A variety of processes are used to recover viscous hydrocarbons, such as heavy oils and bitumen, from reservoirs such as oil sands deposits. Extensive deposits of viscous hydrocarbons exist around the world, including large deposits in the Northern Alberta oil sands that are not susceptible to standard oil well production technologies. One problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially relevant rates at the temperatures and pressures present in the reservoir.
In some cases, such deposits are mined using open-pit mining techniques to extract hydrocarbon-bearing material for later processing to extract the hydrocarbons. Alternatively, thermal techniques may be used to heat the hydrocarbon reservoir to mobilize the hydrocarbons and produce the heated, mobilized hydrocarbons from wells.
One thermal method of recovering viscous hydrocarbons using two vertically spaced horizontal wells is known as steam-assisted gravity drainage (SAGD). Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal, injection well into a viscous hydrocarbon reservoir while mobilized hydrocarbons are produced from a lower, parallel, horizontal, production well that is vertically spaced and near the injection well. The injection and production wells are located close to the bottom of the hydrocarbon deposit to collect the hydrocarbons that flow toward the bottom.
The SAGD process is believed to work as follows. The injected steam initially mobilizes the hydrocarbons to create a steam chamber in the reservoir around and above the horizontal injection well. The term “steam chamber” is utilized to refer to the volume of the reservoir that is saturated with injected steam and from which mobilized oil has at least partially drained. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, in particular, at the margins of the steam chamber where the steam condenses and heats the viscous hydrocarbons by thermal conduction. The mobilized hydrocarbons and aqueous condensate drain, under the effects of gravity, toward the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber and ensure that the production well remains located at the bottom of the steam chamber in an appropriate position to collect mobilized hydrocarbons.
In situ Combustion (ISC) is another thermal method which may be utilized to recover hydrocarbons from underground hydrocarbon reservoirs. ISC includes the injection of an oxidizing gas into the porous rock of a hydrocarbon-containing reservoir to ignite and support combustion of the hydrocarbons around the wellbore. ISC may be initiated using an artificial igniter such as a downhole heater or by pre-conditioning the formation around the wellbores and promoting spontaneous ignition. The ISC process, also known as fire flooding or fireflood, is sustained and the ISC fire front moves due to the continuous injection of the oxidizing gas. The heat generated by burning the heavy hydrocarbons in place produces hydrocarbon cracking, vaporization of light hydrocarbons and reservoir water in addition to the deposition of heavier hydrocarbons known as coke. As the fire moves, the burning front pushes a mixture of hot combustion gases, steam, and hot water, which in turn reduces oil viscosity and the oil moves toward the production well. Additionally, the light hydrocarbons and the steam move ahead of the burning front, condensing into liquids, facilitating miscible displacement and hot waterflooding, which contribute to the recovery of hydrocarbons.
Canadian Patent 2,096,034 to Kisman et al. and U.S. Pat. No. 5,211,230 to Ostapovich et al. disclose a method of in situ combustion for the recovery of hydrocarbons from underground reservoirs, sometimes referred to as Combustion Split production Horizontal well Process (COSH) or Combustion Overhead Gravity Drainage (COGD). The disclosed processes include gravity drainage to a basal horizontal well in a combustion process. A horizontal production well is located in the lower portion of the reservoir. A vertical injection and one or more vertical vent wells are provided in the upper portion of the reservoir. Oxygen-enriched gas is injected down the injector well and ignited in the upper portion of the reservoir to create a combustion zone that reduces viscosity of oil in the reservoir as the combustion zone advances downwardly toward the horizontal production well. The reduced-viscosity oil drains into the horizontal production well under the force of gravity.
Canadian Patent 2,678,347 to Bailey discloses a pre-ignition heat cycle (PIHC) using cyclic steam injection and steam flood methods that improve the recovery of viscous hydrocarbons from a subterranean reservoir using an overhead in situ combustion process, referred to as combustion overhead gravity drainage (COGD). Bailey discloses a method where the reservoir well network includes one or more injection wells and one or more vent wells located in the top portion of the reservoir, and where the horizontal drain is located in the bottom portion of the reservoir.
The use of ISC as a follow up process to SAGD is disclosed in Canadian Patent 2,594,414 to Chhina et al. The disclosed hydrocarbon recovery processes may be utilized in hydrocarbon reservoirs. Chhina discloses a process where a former steam injection well, used during the preceding SAGD recovery process, is used as an oxidizing gas injection well and where another former steam injection well, adjacent to the oxidizing gas injection well, is converted into a combustion gas production well. This results in the horizontal hydrocarbon production well being located below the horizontal oxidizing gas injection well and at least one combustion gas production well being spaced from the injection well by a distance that is greater than the spacing between hydrocarbon production well and the oxidizing gas injection well. Since the process disclosed by Chhina uses at least two wells pairs, ISC is initiated after the production well is sufficiently depleted of hydrocarbons to establish communication between the two well pairs.
Hydrocarbon reservoirs may exist substantially in isolation, or may also exist adjacent to aqueous mobile fluid zone formations that have relatively low bitumen saturation and significant saturations of water. In such deposits, these aqueous mobile fluid zone formations can act as a “thief zone” and have one or more undesirable effects on recovery methods. For example, if this adjacent aqueous mobile fluid zone is in fluid communication with the reservoir being recovered, the adjacent aqueous mobile fluid zone may detrimentally absorb heat which would otherwise be used in the thermal recovery process to produce hydrocarbons.